1. Field of the Invention
This invention pertains to bonded fibrous webs and methods for bonding and rearranging the fiber structure of such webs. It finds particular utility in the bonding and rearranging of nonwoven fabrics for limited use applications. More particularly, it relates to such bonding and rearranging of fibers in a web through the use of high frequency vibrational energy. In its common form, such an application is referred to as ultrasonic bonding although the frequency range is not limited to those above sonic frequencies.
Nonwoven webs and fabrics have gained wide acceptance and are in common use as protective garments, wipers, health care items (such as bed linens, surgical drapes, and the like) and as elements of many other more durable products.
2. Description of the Prior Art
As formed, most nonwoven webs lack sufficient strength for many applications and must be reinforced through the use of adhesives, embossing, activation of thermoplastic material, or by combining with a reinforcing web. In addition to the cost occasioned by each of these steps, the resulting products suffer a loss in one or more desirable properties such as hand, drape, softness, and the like. Therefore, the design of a nonwoven material usually involves a compromise between fabric strength and its other properties.
Recently there have been developed methods of bonding nonwoven webs and increasing their strength properties without necessitating such conventional bonding steps and their inherent drawbacks. These newly developed processes rely on the entanglement of the fibers which make up the web for consolidation and strengthening. Examples include needling, which is described generally in U.S. Pat. Nos. 3,538,564 to Skoler et al issued Nov. 10, 1970; 3,758,926 to Dilo issued Sept. 18, 1973; 3,783,479 to Terry issued Jan. 8, 1974; and 3,827,112 to Betere issued Aug. 6, 1974; for example, and fluid jet entanglement, which is described in U.S. Pat. Nos. 3,129,466 to L'Hommedieu issued Apr. 21, 1964; 3,214,819 to Guerin issued Nov. 2, 1965; 3,485,706 to Evans issued Dec. 23, 1969; 3,042,576 to Harmon et al issued July 3, 1962; 3,081,500 to Griswold et al. issued Mar. 19, 1963; 3,683,921 to Brooks et al issued Aug. 15, 1972; as well as Kalwaites patents 2,862,251 issued Dec. 2, 1958; 3,769,659 issued Nov. 6, 3,800,364 issued Apr. 2, 1974; and others. Needling is inherently a slow process, and the formation and handling of fluid jets requires cumbersome and costly equipment.
Bonding of many diverse materials, including fibrous web structures, by sonic means is rapidly becoming a developed art. Thus, it is known to cause welding of thermoplastic fibers, films, and articles by heat generated due to the frictional forces resulting from vibrations of the sonic horn. Alternatively, the ultrasonic vibrations can be used to promote hydrogen bonding in cellulosic webs or to activate a previously applied adhesive as shown, for example, in U.S. Pat. No. 3,660,186 to Sager et al issued May 2, 1972 which is assigned to the present assignee of this application. In the manufacturing of nonwovens, however, the use of ultrasonics for activation of thermoplastics or for activation of adhesives has tended to suffer the same drawbacks as such bonding obtained by conventional means. That is, the webs produced lack a desirable degree of hand, drape, and softness.